Plasmids from oral streptococci and their role in the physiology and ecology of this group of organisms are under investigation. Within this contest, a multiple antibiotic resistant strain of Streptococcus mutans (19S), originally isolated from the jaw of a pig, is being characterized. This isolate transferred, by conjugation, only its tetracycline resistance to two strains of S. faecalis and to one other strain of S. mutans. This transfer was always accompanied by the appearance of an 8 megadalton plasmid (with a copy number of 1) in the transconjugants. However, such a plasmid has, so far, not been identified in the original S. mutans host, 19S. Presently, we are using the Southern hybridization method, with plasmid DNA from one of the transconjugants as the labeled probe, to ascertain whether the tetracycline resistance gene in the original 19S host is on the chromosome or on a plasmid that cannot be detected by the methods used. Preliminary results suggest that the latter may be the case.